The present invention relates to a continuously variable speed-changing device and, more specifically, to a device for changing the speeds of the front and rear wheels in a four-wheel-drive vehicle, capable of variably driving the front wheels and the rear wheels independently from each other by using a toroidal type continuously variable speed-changing device.
There has heretofore been proposed a four-wheel-drive vehicle equipped with a continuously variable speed-changing mechanism for the front wheels and with a continuously variable speed-changing mechanism for rear wheels in order to variably drive the front wheel drive shafts and the rear wheel drive shafts independently from each other via the continuously variable speed-changing mechanisms, in an attempt to decrease the weight of the four-wheel-drive mechanisms, to avoid the loss of power and to improve the fuel efficiency.
When the four-wheel-drive vehicle turns in accordance with the operation of the steering wheel, there occurs the so-called inside-outside wheel difference that the front wheels turn on the center of rotation with the radii of turn which are on the outer sides of the radii of turn of the rear wheels. The difference in the rotational speed between the front right wheel and the front left wheel is absorbed by a differential for the front wheels provided for the front axles, and the difference in the rotational speed between the rear right wheel and the rear left wheel is absorbed by a differential for the rear wheels provided for the rear axles. When turning, however, the front wheels and the rear wheels travel different distances since the average radius of turn is not the same between the front wheels and the rear wheels. When no countermeasure is taken, like in the four-wheel-drive vehicle of the directly coupled type, therefore, the rotation of the front wheels does not meet the rotation of the rear wheels, giving rise to the occurrence of a so-called tight corner braking phenomenon. The tight-corner braking phenomenon is an undesirable phenomenon impairing smooth turning of the vehicle; i.e., the drive system is greatly twisted due to a difference in the rotation between the front wheels and the rear wheels, the slip and drive of the wheels are repeated making it difficult to smoothly operate the vehicle and, in extreme cases, making it difficult to run the vehicle.
A typical example of the continuously variable speed-changing device in the four-wheel-drive vehicle can be typified by a toroidal type continuously variable speed-changing device disclosed in Japanese Laid-open Patent Publication (Kokai) No. 157151/1993 (JP-A 5-157151). This continuously variable speed-changing device includes an input shaft supported by a casing, and a continuously variable speed-changing mechanism for the front wheels and a continuously variable speed-changing mechanism for the rear wheels that are arranged, spaced apart, in the axial direction of the input shaft. The continuously variable speed-changing mechanism for the front wheels changes the input rotational speed of the input shaft and outputs it to the front wheel drive shafts. The continuously variable speed-changing mechanism for the rear wheels changes the input rotational speed of the input shaft and outputs it to the rear wheel drive shafts. These continuously variable speed-changing mechanisms have been so constituted as to variably drive the front wheels and the rear wheels independently from each other.
The above continuously variable speed-changing mechanism for the front wheels has substantially the same constitution as that of the continuously variable speed-changing mechanism for the rear wheels. Therefore, the constitution of the continuously variable speed-changing mechanism for the front wheels will now be briefly described. The continuously variable speed-changing mechanism for the front wheels includes an input disk which is secured to the input shaft and has one surface in the axial direction thereof formed as an input side recessed surface of an arcuate shape in cross section, and an output disk having one surface in the axial direction thereof formed as an output side recessed surface of an arcuate shape in cross section. The output disk is so disposed as to rotate relative to the input shaft and to surround the outer peripheral surface of the input shaft and has its output side recessed surface disposed so as to be opposed to the input side recessed surface of the input disk in the axial direction. A pair of power rollers are disposed between the input side recessed surface of the input disk and the output side recessed surface of the output disk so as to rotate relative to the input disk and the output disk. The power rollers are supported by trunnions so as to rotate. The axes of rotation of the power rollers are arranged at right angles with the axes of the trunnions that will be described later.
The trunnions are so supported as to rotate about their axes relative to the casing and to move in the axial direction. The axes of the trunnions extend in parallel with each other in the tangential direction relative to the input shaft at symmetrical positions with the input shaft interposed therebetween at an equal distance. On the cross sections of the input disk and of the output disk opposed to each other in the axial direction, the center of arc of the input side recessed surface of the input disk and the center of arc of the output side recessed surface of the output disk are arranged on a common center of arc. Further, the axial centers of the trunnions are arranged to be in agreement with the centers of the corresponding arcs. The axes of the trunnions define tilting axes along which the corresponding power rollers rotate. The power rollers are so constituted as to possess spherically protruded surfaces in the peripheries thereof, the protruded surfaces being brought into a pressed contact (point contact) with the input side recessed surface of the input disk and with the output side recessed surface of the output disk. The trunnions are moved in the axial directions but in the opposite directions relative to each other by the actuators such as hydraulic cylinders, whereby the power rollers rotate about the axes which are tilted by an angle corresponding to the amount of motion of the trunnions and changing of the speed between the input disk and the output disk is performed. According to the toroidal type continuously variable speed-changing device as described above, the rotational forces of the output disks of the continuously variable speed-changing mechanism for the front wheels and of the continuously variable speed-changing mechanism for the rear wheels are output to the front wheels and to the rear wheels independently from each other, to execute the full-time four-wheel drive. Constitutions of trunnions of the toroidal type continuously variable speed-changing device have been disclosed in, for example, Japanese Utility Model Publication (Kokoku) No. 11425/1994 and Japanese Laid-open Patent Publication (Kokai) No. 269039/1997 (JP-A 9-269039).
In the continuously variable speed-changing device, when a steering angle is given to the front wheels, the tilting angle of the power rollers of the continuously variable speed-changing mechanism for the front wheels is differed from the tilting angle of the power rollers of the continuously variable speed-changing mechanism for the rear wheels to drive the four wheels with little loss of power while absorbing the difference in rotational speed between the front wheels and the rear wheels when the vehicle turns, without using the center differential. That is, the tilting angle of the power rollers in the continuously variable speed-changing mechanism for the rear wheels is slightly decreased compared to that of when traveling straight and the tilting angle of the power rollers in the continuously variable speed-changing mechanism for the front wheels is slightly increased compared to that of when traveling straight, so that the rotational speed of the front wheels becomes slightly faster than that of when traveling straight and that the rotational speed of the rear wheels becomes slightly slower than that of traveling straight, thus imparting a difference in the speed between the front wheels and the rear wheels when the vehicle is turning.
In order to cope with the tight corner braking phenomenon that occurs due to a difference in the speed between the front wheels and the rear wheels when a four-wheel-drive vehicle turns, the continuously variable speed-changing device so works that the front wheels run faster than when traveling straight and that the rear wheels run slower than when traveling straight depending upon the steering angle based on a prerequisite that the front wheels and the rear wheels have an equal tire diameter. However, operating the steering wheels at a large steering angle is usually limited to traveling at a low speed. In order to run the rear wheels slower than when traveling straight while the vehicle is turning, therefore, the continuously variable speed-changing mechanism for the rear wheels must have a margin for further tilting the power rollers toward the speed-decreasing side while the vehicle is turning in addition to the range of a maximum tilting angle of the power rollers on the speed-decreasing side of when traveling straight. In order to impart a margin that is not used when traveling straight to the speed-changing range of the continuously variable speed-change mechanism for the rear wheels on the speed-decreasing side, it becomes necessary to further increase a maximum outer diameter of the output disk. An increase in the maximum outer diameter of the output disk means an increase in the weight of the output disk as well as an increase in the force of inertia, affecting the braking performance and driving up the cost. When it is forced to increase the size of the casing with an increase in the size of the output disk, the degree of inconvenience further increases. During the steering operation, further, both the rear wheels and the front wheels must be simultaneously controlled toward the-speed-decreasing side and toward the speed-increasing side with the speed of traveling straight as a reference. Therefore, the control operation becomes complex and may not be stably executed.
It is an object of the present invention to provide a novel device for changing the speeds of the front and rear wheels in a four-wheel-drive vehicle, which prevents the occurrence of tight-corner braking phenomenon and ensures the smooth turn-traveling at the time of turn-traveling of the vehicle, by a relatively simple control operation without changing the constitution.
Another object of the present invention is to provide a novel device for changing the speeds of the front and rear wheels in a four-wheel-drive vehicle, which prevents the occurrence of tight-corner braking phenomenon when the vehicle turns and enables the vehicle to smoothly turn by independently outputting the rotational forces to the front wheels and to the rear wheels from the output disk of the continuously variable speed-changing mechanism for the front wheels and from the output disk of the continuously variable speed-changing mechanism for the rear wheels, to accomplish the full-time four-wheel drive operation without the need of imparting additional margin that is not used when traveling straight to the speed-change range of the continuously variable speed-changing mechanism for the rear wheels on the speed-decreasing side to allow a relatively simple control operation.
According to the present invention, there is provided a device for changing the speeds of the front and rear wheels in a four-wheel-drive vehicle including a continuously variable speed-changing mechanism for the front wheels, which changes the input rotational speed and outputs it to the front wheel drive shafts, and a continuously variable speed-changing mechanism for the rear wheels, which changes the input rotational speed and outputs it to the rear wheel drive shafts, so that the front wheels and the rear wheels can be driven at continuously variable speeds independently of each other due to the continuously variable speed-changing mechanisms, the device for changing the speeds of the front and rear wheels comprising:
a detector means for detecting the operation conditions of the vehicle;
a steering angle sensor for detecting the steering angle of the vehicle; and
a controller for variably controlling the speeds of the continuously variable speed-changing mechanism for the front wheels and the continuously variable speed-changing mechanism for the rear wheels based on the operation conditions of the vehicle detected by the detector means;
the controller setting a target speed-changing ratio obtained from the vehicle operating conditions to be a target speed-changing ratio for the rear wheels in the continuously variable speed-changing mechanism for the rear wheels, and setting a speed-changing ratio obtained from the target speed-changing ratio and from a steering angle detected by the steering angle sensor to be a target speed-changing ratio for the front wheels in the continuously variable speed-changing mechanism for the front wheels.
It is desired that the controller obtains the target speed-changing ratio for the front wheels in compliance with a predetermined equation using the target speed-changing ratio and the steering angle as variables.
It is desired that the controller obtains the target speed-changing ratio for the front wheels from the target speed-changing ratio and the steering angle detected by the steering angle sensor based on a map formed in advance by using, as coordinate axes, the steering angle and the target speed-changing ratio for the front wheels, that corresponds to the steering angle and is determined by the target speed-changing ratio as a reference.
It is desired that the controller obtains the target speed-changing ratio for the front wheels, that corresponds to the steering angle, over a whole range of the target speed-changing ratio, from an approximate equation using the steering angle only as a variable, according to the steering angle detected by the steering angle sensor.
It is desired that the controller obtains the target speed-changing ratio for the front wheels by obtaining a corrected speed-changing ratio for obtaining the target speed-changing ratio for the front wheels, that corresponds to the steering angle detected by the steering angle sensor, in compliance with a predetermined equation, and by adding the corrected speed-changing ratio to the target speed-changing ratio for the rear wheels.
It is desired that the controller obtains the target speed-changing ratio for the front wheels by obtaining a corrected speed-changing ratio corresponding to the steering angle detected by the steering angle sensor based on a map formed in advance by using, as coordinate axes, the steering angle and the corrected speed-changing ratio for obtaining the target speed-changing ratio for the front wheels, that corresponds to the steering angle and is determined by the target speed-changing ratio as a reference, and by adding the corrected speed-changing ratio to the target speed-changing ratio for the rear wheels.
It is desired that the controller obtains the target speed-changing ratio for the front wheels, by obtaining a corrected speed-changing ratio for obtaining the target speed-changing ratio for the front wheels, that corresponds to the steering angle, over a whole range of the target speed-changing ratio, from an approximate equation using the steering angle only as a variable, according to the steering angle detected by the steering angle sensor, and by adding the corrected speed-changing ratio to the target speed-changing ratio for the rear wheels.
It is desired that each of the continuously variable speed-changing mechanism for the front wheels and the continuously variable, speed-changing mechanism for the rear wheels is constituted by a toroidal type continuously variable speed-changing device comprising an input disk for receiving a driving force, an output disk disposed being opposed to the input disk and is drive-coupled to the front wheel drive shafts or to the rear wheel drive shafts, and power rollers disposed between the input disk and the output disk so as to rotate in a tilted manner and change contact points to the input disk and to the output disk to continuously change the rotational speed of the input disk and to transmit the rotation to the output disk.
It is desired that the controller sets a target tilting angle of the power rollers for the front wheels in the continuously variable speed-changing mechanism for the front wheels and a target tilting angle of the power rollers for the rear wheels in the continuously variable speed-changing mechanism for the rear wheels respectively corresponding to the target speed-changing ratio for the front wheels and the target speed-changing ratio for the rear wheels, to control the continuously variable speed-changing mechanism for the front wheels and the continuously variable speed-changing mechanism for the rear wheels based on the target tilting angle for the front wheels and the target tilting angle for the rear wheels.